A need for real time solder paste process control is critical due to the dynamic nature of solder paste. The rheology and thixotropic properties of solder paste can change dramatically during manufacture which can cause significant problems in the printablity of circuit boards using the solder paste. These changes are dependent on the manufacturing environment, the characteristics and age of the paste, and the previous treatment of the paste. A high humidity environment can cause an increase in slump and an increase in the occurrence of solder balls due to moisture absorption. Solder paste can also increase or decrease in viscosity with age. Changes in the flux material can impact the rheologic nature of the paste due to excessive build-up of reaction products between the flux activators and the metal oxides.
Methods currently used for measuring the rheology of solder paste include the use of a viscometer. A viscometer can measure the viscosity of solder paste at different shearing rates. In this way the viscosity of a solder paste can be tracked from a reference shear rate and the thixotropic character of the paste can be tracked by calculating the change in viscosity from the change in shear rate. Viscometer systems allow a manufacturer to calculate both viscosity and thixotropic behavior. However, these systems are primarily designed to make bulk rheologic measurements as an incoming inspection tool and typically require a large amount of paste for an accurate measurement.
A related patent is U.S. Pat. No. 5,656,933 issued Aug. 12, 1997. U.S. Pat. No. 5,103,181 discloses a composition and monitoring process that uses impedance measurements. U.S. Pat. No. 4,939,469 discloses a method for evaluating printed circuit boards using the impedance spectra of the board. U.S. Pat. Nos. 3,482,161; 3,440,529 and 3,448,380 all use spectroscopic analysis for analyzing samples.